This application relates to acoustic pulse-echo ranging systems similar to those described in our U.S. Pat. No. 4,831,565, issued May 16, 1989.
That patent described, inter alia, a technique for determining whether a transducer in such a system is operating normally, by testing the amplitude of the signal received from the transducer a predetermined time after the application of a transmit pulse while the high-Q transducer should still be ringing at high amplitude as a result of the application of the transmit pulse. This technique could theoretically be used to detect whether the transducer is submerged in material whose level it is monitoring, since such submergence will normally severely damp the ringing of the transducer.
Acoustic transducers used in such applications usually have a skirt surrounding a downwardly facing radiating surface of the transducer, which traps air as the transducer becomes submerged. Radiation from the transducer will be reflected backwards and forwards between the surface being ranged and the transducer face with the result that the ringing of the transducer will usually become more sustained as the transducer becomes submerged. This has been used as a means for detecting submersion of the transducer, by detecting whether the initial rate of decrease of the ringing amplitude following a transmit pulse has been slower than a predetermined threshold rate, and/or whether the amplitude of ringing at a predetermined interval after the transmit pulse remains above a preset threshold.
In practice, it is found that these techniques are not always reliable because of substantial variations in the degree of reflectivity of the surfaces that can occur with different materials and circumstances.
It is an object of the present invention to provide a technique for detecting submergence of transducers which provide more consistent indications, and is less influenced by variations in surface reflectivity.
According to the invention, there is provided a method for detecting actual or incipient submersion of an electroacoustic transducer facing downwardly towards a surface of fluent material to be sensed in a pulse-echo acoustic ranging system, the transducer having a skirt surrounding a downwardly facing radiating surface to maintain an air space beneath the radiating surface even under submergence conditions, and the transducer being electrically energized to emit pulses of acoustic energy from the radiating surface, the system processing an electrical output received from the transducer following a pulse by repeatedly sampling it to obtain a response profile;
wherein samples from an initial portion of the response of the transducer during a ring-down period following the transmit pulse are summed in a manner such as to eliminate from the sum portions of each sample due to echo responses above a threshold determined on the basis of preceding samples, and the sum is compared with predetermined threshold to determine whether submergence is indicated.
Conveniently, the echo response threshold for a sample is an amplitude which if exceeded would produce an upturn in the response profile between successive samples, e.g. a sample larger than the lowest preceding sample will exceed the threshold. Such a summation may be achieved simply by summing successive samples, but substituting the value of each sample, whose value exceeds the lowest sample value previously detected, by that lowest value.